PIM WG IJ. Wijnands
Internet-Draft A. Boers
Intended status: Standards Track E. Rosen
Expires: July 19, 2009 Cisco Systems, Inc.
January 15, 2009
The RPF Vector TLVdraft-ietf-pim-rpf-vector-08
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Abstract
This document describes a use of the PIM Join Attribute as defined in
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Internet-Draft The PIM RPF Vector TLV January 20091. Specification of Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Introduction
It is sometimes convenient to distinguish the routers of a particular
network into two categories: "edge routers" and "core routers". The
edge routers attach directly to users or to other networks, but the
core routers attach only to other routers of the same network. If
the network is MPLS-enabled, then any unicast packet which needs to
travel outside the network can be "tunneled" via MPLS from one edge
router to another. To handle a unicast packet which must travel
outside the network, an edge router needs to know which of the other
edge routers is the best exit point from the network for that
packet's destination IP address. The core routers, however, do not
need to have any knowledge of routes which lead outside the network;
as they handle only tunneled packets, they only need to know how to
reach the edge routers and the other core routers.
Consider, for example, the case where the network is an Autonomous
System (AS), the edge routers are EBGP speakers, the core routers may
be said to constitute a "BGP-free core". The edge routers distribute
BGP routes to each other, but not to the core routers.
However, when multicast packets are considered, the strategy of
keeping the core routers free of "external" routes is more
problematic. When using PIM Sparse-Mode (PIM-SM) [RFC4601], PIM
Source-Specific Mode (PIM-SSM) [RFC4607] or Bidirectional PIM (BIDIR-
PIM) [RFC5015] to create a multicast distribution tree for a
particular multicast group, one wants the core routers to be full
participants in the PIM protocol, so that multicasting can be done
efficiently in the core. This means that the core routers must be
able to correctly process PIM Join messages for the group, which in
turn means that the core routers must be able to send the Join
messages towards the root of the distribution tree. If the root of
the tree lies outside the network's borders (e.g., is in a different
AS), and the core routers do not maintain routes to external
destinations, then the PIM Join messages cannot be processed, and the
multicast distribution tree cannot be created.
In order to allow PIM to work properly in an environment where the
core routers do not maintain external routes, a PIM extension is
needed. When an edge router sends a PIM Join message into the core,
it MUST include in that message a "Vector" which specifies the IP
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Internet-Draft The PIM RPF Vector TLV January 2009
address of the next edge router along the path to the root of the
multicast distribution tree. The core routers can then process the
Join message by sending it towards the specified edge router (i.e.,
toward the Vector). In effect, the Vector serves as an attribute,
within a particular network, for the root of the tree.
This document defines a new TLV in the PIM Join Attribute message
[RFC5384]. It consists of a single Vector which identifies the exit
point of the network.
3. Use of the RPF Vector TLV
Before a router can start forwarding multicast packets, it is
necessary to build a forwarding tree by sending PIM Joins hop by hop.
Each router in the path creates a forwarding state and propagates the
Join towards the root of the forwarding tree. The building of this
tree is receiver driven. See Figure 1.
------------------ BGP -----------------
| |
[S]---( Edge 1)--(Core 1)---( Core )--(Core 2)---( Edge 2 )---[R]
<--- (S,G) Join
Figure 1
In this example, the 2 edge routers are BGP speakers. The core
routers are not BGP speakers and do not have any BGP distributed
routes. The route to S is a BGP distributed route, hence is known to
the edge but not to the core. The Edge 2 router determines the
interface leading to S, and sends a PIM Join to the upstream router.
In this example, though, the upstream router is a core router, with
no route to S. Without the PIM extensions specified in this document,
the core router cannot determine where the send the Join, so the tree
cannot be constructed.
To allow the core router to participate in the construction of the
tree, the Edge 2 router will include an attribute field in the PIM
Join. In this example, the Attribute field will contain the IP
address of Edge 1. Edge 2 then forwards the PIM Join towards Edge 1.
The intermediate core routers do their RPF (Reverse Path Forwarding)
check [RFC4601] on the Attribute (IP address of Edge 1) rather than
the Source, this allows the tree to be constructed.
3.1. Attribute and shared tree joins
In the example above we build a source tree to illustrate the
attribute behavior. The attribute is however not restricted to
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Internet-Draft The PIM RPF Vector TLV January 2009
source tree only. The tree may also be constructed towards a
Rendezvous Point (RP) IP address. The RP IP address is used in a
similar way as the Source in the example above. PIM Attribute
procedures defined for sources are equally applicable to (*,G) and
(*,*,RP) joins unless otherwise noted.
3.2. Attribute and Bootstrap messages
The RPF vector does not apply to BSR bootstrap messages. To allow
BSR messages to be forwarded across a core where the BSR IP address
is not routable in the core a solution needs to be developed for BSR.
3.3. The Vector Attribute3.3.1. Inserting a Vector Attribute in a Join
In the example of Figure 1, when the Edge 2 router looks up the route
to the source of the multicast distribution tree, it will find a BGP-
distributed route whose "BGP next-hop" is Edge 1. Edge 2 then looks
up the route to Edge 1 to find interface and PIM adjacency which is
the next hop to the source, namely Core 2.
When Edge 2 sends a PIM Join to Core 2, it includes a Vector
Attribute specifying the address of Edge 1. Core 2, and subsequent
core routers, will forwarding the Join along the Vector (i.e, towards
Edge 1) instead of trying to forward it towards S.
Whether an attribute is actually needed depends on whether the Core
routers have a route to the source of the multicast tree. How the
Edge router knows whether or not this is the case (and thus how the
Edge router determines whether or not to insert an attribute field)
is outside the scope of this document.
3.3.2. Processing a Received Vector Attribute
When processing a received PIM Join which contains a Vector
Attribute, a router MUST first check to see if the Vector IP address
is one of its own IP addresses. If so, the Vector Attribute is
discarded, and not passed further upstream. Otherwise, the Vector
Attribute is used to find the route to the source, and is passed
along when a PIM Join is sent upstream. Note that a router which
receives a Vector Attribute MUST use it, even if that router happens
to have a route to the source. A router which discards a Vector
Attribute MAY of course insert a new Vector Attribute. This would
typically happen if a PIM Join needed to pass through a sequence of
Edge routers, each pair of which is separated by a core which does
not have external routes. In the absence of periodic refreshment,
Vectors expire along with the corresponding (S,G) state.
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Internet-Draft The PIM RPF Vector TLV January 20093.3.3. Vector Attribute and Asserts
A PIM Assert message includes the routing protocol's "metric" to the
source of the tree. This information is used in the selection of the
assert winner. If a PIM Join is being sent towards a Vector, rather
than towards the source, the Assert message MUST have the metric to
the Vector instead of the metric to the source. The Assert message
however does not have an attribute field and does not mention the
Vector.
A router may change its upstream neighbor on a particular multicast
tree as the result of receiving Assert messages. However a Vector
Attribute MUST NOT be sent in a PIM Join to an upstream neighbor
which is chosen as the result of an Assert processing, if that
neighbor is different than the original upstream neighbor.
Reachability of the Vector is only guaranteed by the router that
advertises reachability to the Vector in its IGP. If the assert
winner upstream is not the real preferred next-hop, it is possible
that the assert winner does not know the path to the Vector. In the
worst case the assert winner has a route to the Vector that is on the
same interface where the assert was won. That will point the RPF
interface to that interface and will result in the O-list being NULL.
The Vector attribute therefore MUST NOT be inserted if the RPF
neighbor was chosen via an assert process and the RPF neighbor is
different from the RPF neighbor that would have been selected via the
local routing table. In all other cases the Vector MUST be included
in the Join message.
3.3.4. Vector Attribute and Join suppression
If a router receives a PIM join on the upstream LAN interface for a
particular multicast state, join suppression may be applied if that
PIM join is targeted to the same upstream neighbor. Which router(s)
will suppress their PIM join is dependant on timing and is
unpredictable. Downstream routers on a LAN MAY include different RPF
vectors in the PIM joins. Therefore an upstream router on that LAN
may receive and use different RPF vectors over time to reach the
destination (depending on which downstream router(s) suppressed their
Join). To make the upstream router behavior more predictable the RPF
vector address MUST be used as additional condition to the join
suppression logic. Only if the RPF vector in the PIM join matches
the RPF vector in the multicast state, the suppression logic is
applied. It is also possible to disable join suppression on that
LAN.
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